The amount of heat per unit area generated by a semiconductor device to be mounted onto a wiring substrate has increased due to the micro-wiring and downsizing of the semiconductor device. It is therefore desired to form a wiring substrate by a material having higher thermal conductivity and a heat radiating capability. Conventionally, an alumina sintered body has been used as the material for the wiring substrate; however, because of the insufficient thermal conductivity of the alumina sintered body, the possibility of using an aluminum nitride sintered body, which has higher thermal conductivity, is being considered.
In order to produce a wiring substrate by using a sintered nitride ceramic substrate, a typical example of which is the aluminum nitride sintered body, it is necessary to form metal wiring on a surface of the nitride ceramic sintered body. Examples of the method for forming metal wiring include: a thick-film method in which a metal paste is applied and fired; and a thin-film method in which a metal thin film is formed by vapor deposition. Especially in the uses where the heat radiating capability is required, a large amount of electric current is often needed. As the thickness of the film formed by the thin-film method limits the amount of electric current permitted to flow, the thick-film method is favorably adopted. On the other hand, in the thick-film method, high-melting-point metal such as tungsten and molybdenum is often used to form metal wiring; and in this case, there is a problem that wiring resistance is increased.
The following are known as an industrial method for forming metal wiring by the thick-film method: a co-firing method and post-firing method in which a paste containing a high-melting-point metal powder is used. The co-firing method is a method in which to apply a high-melting-point metal paste onto a ceramic green sheet and fire them to thereby carry out sintering of the ceramic and firing of the high-melting-point metal concurrently. This co-firing method has characteristics that although it enables a solidly adhered metal layer to be formed, it is difficult to form a metal pattern with high dimensional precision due to the shrinkage of the ceramic following the sintering thereof. The post-firing method is a method in which to apply a high-melting-point metal paste onto a pre-sintered ceramic substrate and thereafter fire it. This method basically does not cause such a problem of dimensional precision as mentioned above. However, it has been difficult heretofore, by the post-firing method, to attain high bonding strength (adhesion strength) of the metal layer on the nitride ceramics. These days, there is being developed a post-firing method that enables formation of a high-melting-point metal layer adhered with high bonding strength even on the nitride ceramics (see Patent Document 1). However, there has not yet been established industrially a technique of forming a metal layer on a sintered nitride ceramic substrate by using different metal pastes that can lower the wiring resistance more.
With an aim to solve this problem, Patent Document 2 discloses an aluminum nitride substrate comprising a metallized layer formed by applying a paste containing a Ag—Cu alloy as the main component and titanium hydride as the secondary component, onto a sintered aluminum nitride substrate, and firing it.
In addition, Patent Document 3 discloses a metal powder composition for metallization for forming a metallized layer on a ceramic substrate, the metal powder composition comprising Cu powder and Ti powder as the main component and at least one selected from Ag, Al, and Zr as the secondary component, wherein the above main component accounts for 90 to 99.5% by weight %, and the above secondary component accounts for 0.5 to 10% by weight %; and discloses a production method of a metallized substrate using this metal powder composition for metallization.